1. Field of the Invention
This invention relates to an image forming apparatus, and, more particularly, to an improvement of an image forming apparatus which converts an image of an original into an electric signal through photoelectric conversion, performs a predetermined process on the electric signal and then forms an image corresponding to the original's image on a sheet in accordance with the processed signal.
2. Description of the Related Art
In making a copy by a conventional image forming apparatus, e.g., a copying machine, first, an original image disposed on an original table is exposed to light and scanned by a light source, such as an exposure lamp. The original image is formed, by light from the light source, on a photoconductive drum through a plurality of mirrors and lenses, and is then developed with a developer by a developing unit. This developed image is transferred on to a sheet fed from the outside of the main body of the copying machine and is fixed, which completes the copying process. The sheet with a copied image is then discharged outside of the main body.
With such a conventional copying machine, it is well known that lenses are moved to change the length of the light path by a predetermined amount in order to vary image magnification. In this case, due to variation in lenses through manufacturing processes, the focal lengths of individual lenses slightly differ from one another. To attain an accurate magnification, therefore, the displacement of the lenses should be changed in proportion to the focal lengths of the lenses. Conventionally, the focal length of each lens is measured in advance and the lens' displacement is determined on the basis of the measured data. Due to the mentioned slight variation in lenses, however, this method requires many processes for the measurement.
In a so-called digital copying machine which converts an image of an original into an electric signal through photoelectric conversion, performs a predetermined process on the electric signal and then forms an image corresponding to the original's image on a sheet in accordance with the processed signal, the magnification change by an optical system and the magnification change by an electric signal processing are used together. In the digital copying machine, therefore, it is necessary to accurately compensate for variation in the focal lengths of lenses. Accordingly, with the use of the aforementioned method, it takes a great deal of time for measuring the lenses' focal lengths and adjustment of the machine based on the measured data, thus making the machine assembling difficult. In addition, the conventional method deteriorates the magnification accuracy.